1. Field of the Invention
The present invention relates to thermally responsive switches for protecting electric motors against thermal damage by opening an electrical circuit of the motors and, more particularly, to such switches in which the element that actuates a movable contact is thermally responsive and snap-acting to move and return with snap-action depending upon a high or low degree of temperature.
2. Discussion of the Prior Art
Conventionally, a thermally responsive snap-acting element 1 is formed into concave shape having a movable contact at its one end 1a and cantilever mounted to a stationary support 2 through the opposite end 1b of said end 1a as shown in FIG. 1. Or otherwise, in addition to said ends 1a, 1b the snap-acting element 1 has a central hole 1.5 into which a pin 3 having a lower end stop 4 is loosely inserted with said end 1b engaged in a side groove 5 made in a stationary support 6 to allow limited vertical traveling of said element 1 as shown in FIGS. 3 and 4. For the former shown in FIG. 1, with the movable contact 1a engaging a fixed contact 7, the contact pressure between the two contacts 1a, 7 often fails to be sufficient because buckling frequently occurs in the proximity of the end portion, i.e., the portion 1c as shown in FIG. 2, since the snap-acting element 1 is cantilever mounted to the stationary support 2. The term "buckling" in this description means that the portion 1c of the snap-acting element 1 is plastically bent by the pressure exerted between the two contacts 1a and 7 before snapping to open the contacts 1a and 7.
For the latter, with a two-point supporting configuration shown in FIG. 3, the buckling hazard is eliminated. However, a disadvantage is caused from the fact that idling clearances exist longitudinally to the element 1 between the element 1 and supports (generally indicated by the numerals 3 and 6). Accordingly, before snapping, the upper central face of said element 1 contacts the pin 3, while the end 1b contacts the lower side 5.1 within the groove 5 as shown in FIG. 3. But when snapped as shown in FIG. 4, the lower central face of the element 1 is forced to move for contacting the stop 4 because of the welding-like force created by arc flame which can prevent the movable contact 1a from separating the fixed contact 7, while the end 1b travels for contacting the upper side 5.2 within said groove 5. As a result, the movable contact 1a approaches the fixed contact 7 by the displacement corresponding to that by which the element 1 lowered. Thus, the required contact-open gap between the contact 1a and 7 is partly lost due to the idling clearances existing in the portion at 3 and 5. For example, assuming the snap-acting element 1 is approximately 15 (mm) long, the contact-open gap narrows up to approximately 0.4 (mm) at its moving end since the snap-acting element 1 bends, but does not move the contact until completely snapped. If the tolerance of said idling clearances on the production process is within 0.1 (mm), the contact-open gap at the point of complete snap becomes as narrow as approximately 0.2 (mm) to that of having no clearance so as to become unacceptable as a contact. On the other hand, when the snap-acting element 1 is rigidly fixed to regulate the idling movement both with the end 1b rigidly fixed to the stationary support 6 and with the pin 3 tightly press fitted into the hole 1.5, however, such stress is set up in said element 1 as to tend curbing its snap-action movement. Accordingly, without permitting the idling, it is not possible for the snap-acting element 1 to move and return with snap action at the desired temperature.